Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system, a process referred to as exhaust gas recirculation (EGR), to reduce regulated noxious emissions. Turbocharged engines may include a low-pressure EGR (LP EGR) system, a high-pressure EGR (HP EGR) system, or both. The LP EGR system diverts exhaust gases after the gases pass through the turbine of the turbocharger and injects the gases before the compressor, while the HP EGR system diverts exhaust gases before the turbine and injects the gases after the intake throttle. An EGR valve may be controlled in either example to achieve a desired intake air dilution, the desired intake air dilution based on engine operating conditions to maintain desirable combustion stability of the engine while providing emissions and fuel economy benefits.
An engine system may also utilize another separate approach to reduce noxious emissions that includes increasing exhaust gas temperatures by operating the engine rich (e.g., increasing the ratio of fuel to air delivered to cylinders of the engine) to generate high levels of engine-out carbon monoxide (CO), hydrogen (H2), and hydrocarbons and at the same time pumping air (herein referred to as secondary air injection or SAI) into the exhaust manifold upstream of an emissions control device (such as an exhaust catalyst). The air pumped into the exhaust manifold may react with the exhaust gases generating an exothermic reaction. As a result, rapid heating of the emissions control device may be achieved and performance of the emissions control device increases, resulting in decreased noxious emissions.
However, the inventors herein have recognized issues with the above approaches. An engine system including only a LP EGR system has a long transport delay, as the exhaust gases travel through the turbocharger compressor, high pressure air induction plumbing, charge air cooler, and intake manifold before reaching the combustion chamber. As a result, it may be difficult to provide the desired amount of EGR to the cylinders, particularly during transient conditions. This is due to the fact that, by the time the EGR reaches the cylinder, the engine speed/load state may have changed and another EGR rate may be desired. An engine system including only a HP EGR system has a reduced EGR delivery time, but the percentage of EGR gases compared to intake gases may be limited and the ability to provide sustained EGR delivery may be compromised by low exhaust pressures. An engine system including a both a LP EGR system and a HP EGR system may experience energy losses due to increased cooling of hot exhaust gases and an increased complexity of valve arrangement. Additionally, the ability to supply secondary air injection to the exhaust system may be controlled by an additional system separate from the LP and EGR systems, further increasing the complexity of the engine system arrangement. Further still, EGR gases are often cooled via an EGR cooler in order to reduce a risk of engine and/or compressor degradation due to injection of hot EGR gases into the intake system. As a result, thermal energy of the hot EGR gases is wasted (e.g., the thermal energy is absorbed by coolant circulating through the EGR cooler) and the engine system may experience increased energy losses due to increased engine load (e.g., increased fan speeds, increased coolant circulation and pump speeds, etc.) in order to cool the EGR gases via the EGR cooler.
In one example, the issues described above may be addressed by a method for flowing intake air through a heat exchanger and selectively to each of an intake system and an exhaust system; flowing exhaust gas through an energy recovery device coupled with the heat exchanger, through the heat exchanger, and selectively to each of the intake and exhaust system; and adjusting the flow of intake air and exhaust gas through the heat exchanger in response to an output of the energy recovery device. In one example of the heat exchanger, exhaust gases may selectively flow from both upstream and downstream of an emissions control device into the heat exchanger and through the energy recovery device. Additionally, compressed intake air may selectively flow from downstream of a compressor and into the heat exchanger. The energy recovery device may receive thermal energy from the hot exhaust gases entering the heat exchanger and convert the thermal energy into electrical energy. The exhaust gases may then travel through the remainder of the heat exchanger where a portion of the thermal energy remaining in the exhaust gases may transfer to fresh air within the heat exchanger, resulting in an increased temperature of the fresh air. The fresh air may then be provided to the exhaust system in the form of secondary air injection, with the increased temperature of the fresh air enabling a more efficient exothermal reaction between the fresh air and the engine exhaust gases, or returned to the intake system upstream of the compressor to decrease compressor surge. The exhaust gases exiting the heat exchanger may be provided to the intake system in the form of LP EGR or HP EGR, with the decreased temperature of the exhaust gases enabling a more efficient compression for increased compressor and/or engine performance. In this way, the output of the heat exchanger may be controlled to supply low pressure LP EGR, HP EGR, secondary air injection, air for compressor surge control, and exhaust energy recovery. By controlling the temperature of the gases within the heat exchanger and the ratio of the gases output by the heat exchanger, the heat exchanger can be utilized with a wide variety of engine operating conditions to increase engine performance and lower emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.